Compressed air storing tank by bedrock cavern

ABSTRACT

A compressed gas storage tank  10  utilizes a rock-bed cavity  11  in which a bentonite slurry is fed into an underground cavity  11  formed in a rock-bed, a forcibly fed compressed gas is stored in said rock-bed cavity in a state in which the compressed gas is loaded with a pressure load of the bentonite slurry from the underside of the compressed gas, the bentonite slurry in the rock-bed cavity  11  is of a dual layer structure consisting of an upper layer composed of a light bentonite slurry  30  mixed with a filling-up material invading into and filling up a void and a crack formed in an inner wall surface and a lower layer composed of a heavy bentonite slurry  13  mixed with a high specific gravity fine powder as a load condition material. The filling-up effect of the bentonite slurry secures sufficient liquid-tightness and air-tightness in the ceiling part of the rock-bed cavity, making it possible to efficiently and economically store compressed gasses such as compressed air or natural gas without allowing them to escape.

TECHNICAL FIELD

[0001] The present invention relates to a compressed gas storage tankutilizing a rock-bed cavity, in which a bentonite slurry (drilling mudor clay base mud) is fed into an underground cavity formed in arock-bed, and compressed gasses such as air or natural gas, which havebeen forcibly fed into the underground cavity through a compressed gaspipe, is stored in the rock-bed cavity in a state in which thecompressed gas is loaded with a pressure load of the bentonite slurryfrom the underside of the compressed gas.

BACKGROUND ART

[0002] For example, a compressed air storage tank (CAES) used for acompressed air storage/gas turbine power generation is constituted suchthat a compressed air is stored in a closed space formed in theunderground.

[0003] It is, in general, difficult to construct or to install a tankfor storing a compressed gas such as a compressed air or natural gasconsiderably deep in the underground. In view of the foregoing,compressed gas storing tanks are proposed, for example, by JapanesePatent Application Laid-Open Nos. 09-154244 and 11-153082, in which anunderground cavity or a abandoned mine formed in a rock-bed, such as aself-sustaining hard rock, by means of excavation operation is utilizedas it is.

[0004] On the other hand, in a tank utilizing such a rock-bed cavity,since a compressed gas tends to leak through a crack or the like formedin a rock-bed at the time of storing the compressed gas, itsair-tightness needs to be improved. As a method for increasing theair-tightness of the rock-bed cavity, there can be contemplated a methodfor forming a concrete layer over an inner wall surface of a rock-bedcavity, a method for adhering a rubber sheet, a method for employing awater sealing tunnel, or a method in which a bentonite slurry is fedinto a rock-bed cavity and a mud cake is formed on an inner wall surfaceby muddy component such as bentonite contained in the bentonite slurryso that a void and a crack formed in the rock-bed is blocked with thismud cake.

[0005] According to this compressed air storage tank mentioned above, aworking fluid composed of a bentonite slurry and the like is suppliedinto an air storage chamber from the underside thereof, which chamber isa closed space for storing a compressed air, for example through aconnected passageway between the air storage chamber and a storagereservoir provided at an upper location of the air storage chamber, andthe working fluid is flowed between the air storage chamber and thestorage reservoir in accordance with a storing quantity of thecompressed air, so that the compressed air is stored in a state in whichthe compressed air is loaded with a hydrostatic pressure of the workingfluid from the underside thereof During night time when a compressed airis fed and stored utilizing surplus electric power, the working fluid ismoved to the storage reservoir as the storing quantity of the compressedair is increased. During day time when the compressed air is used forpower generation, the working fluid is moved to the air storage chamberas the storing quantity of the compressed air is reduced.

[0006] However, the conventional method for increasing the air-tightnessof the rock-bed cavity utilizing a bentonite slurry has shortcomings inthat air and gas tend to remain particularly at the top end part of therock-bed cavity at the time of filling a bentonite slurry and a stablemud cake is difficult to make, thus making it unable to fully block thevoid and crack in some cases. Moreover, self-maintenance ofair-tightness, which is performed, for example, during operation of thecompressed gas storage tank, by repeatedly creating a fluid tight stateutilizing a bentonite slurry and an air-tight state utilizing acompressed gas is not sufficiently performed in some cases, particularlyat the top end part of the rock-bed cavity where the compressed gastends to remain.

DISCLOSURE OF THE INVENTION

[0007] The present invention has been made by paying attention to suchconventional problems as just mentioned above. It is, therefore, anobject of the present invention to provide a compressed gas storage tankutilizing a rock-bed cavity in which a top end part of a rock-bed cavityis surely blocked to realize a sufficient air-tightness, and in whichself-maintenance of air-tightness, which is performed by repeatedlycreating a fluid tight state and an air-tight state, can surely beperformed at the top end part of the rock-bed cavity, and therefore acompressed gas can be stored in a stable condition without allowingescape of the compressed gas.

[0008] The present invention has achieved the above object by providinga compressed gas storage tank utilizing a rock-bed cavity in which abentonite slurry is fed into an underground cavity formed in a rock-bed,and a compressed gas forcibly fed to the underground cavity through acompressed gas pipe is stored in the rock-bed cavity in a state in whichthe compressed gas is loaded with a pressure load of the bentoniteslurry from the underside of the compressed gas, wherein the compressedgas pipe is open downward from a top portion of the rock-bed cavity, andthe bentonite slurry in the rock-bed cavity is of a dual layer structureconsisting of an upper layer composed of a light bentonite slurry mixedwith a filling-up material (for example, a lost circulation inhibitor)invading into and filling up a void and a crack formed in an inner wallsurface and having a specific gravity of 1.05 to 1.20, and a lower layercomposed of a heavy bentonite slurry mixed with a high specific gravityfine powder as a load control material and having a specific gravity of1.20 to 2.0 (the invention as defined in claim 1).

[0009] Moreover, according to the compressed gas storage tank of thepresent invention, it is is preferred that the light bentonite slurry isforcibly fed through the compressed gas pipe into the rock-bed cavity inwhich the bentonite slurry is filled (the invention as defined in claim2).

[0010] Furthermore, according to the compressed gas storage tank of thepresent invention, it is also preferred that the rock-bed cavity is anunderground cavity extending laterally with its ceiling part exhibitinga rising gradient toward the top part which is a connecting portion withthe compressed gas pipe (the invention as defined in claim 3).

[0011] Moreover, according to the compressed gas storage tank of thepresent invention, it is also preferred that the heavy bentonite slurryis fed into the rock-bed cavity through a vertical shaft formed in theunderground and the vertical shaft has associated facilities such as areverse osmosis membrane water-generating pipe and a deep aeration pipejuxtaposed thereto (the invention as defined in claim 4).

[0012] The above-mentioned light bentonite slurry is a muddy waterhaving a specific gravity of 1.05 to 1.20 obtained by adding, forexample, a powder of calcium carbonate having an average grain diameterof about 10 to 20 microns, as a filler, to a bentonite slurry. Thislight bentonite slurry is mixed with, for example, a lost circulationinhibitor (LCM) as a filling-up material serving as a core at the timeof formation of a mud cake for blocking a void and a crack formed in arock-bed. Owing to the presence of the filler and the filling-upmaterial, the mud cake is surely and firmly formed at the void and thecrack of the rock-bed, thereby ensuring a stable air-tightness.

[0013] The above-mentioned heavy bentonite slurry is a suspensionobtained by mixing, for example, a bentonite slurry with a high specificgravity fine powder such as barite and hematite as a load adjustingmaterial in such a stable condition that it may hardly precipitate. Theheavy bentonite slurry is a muddy water having a specific gravity ofabout 1.2 to 2.0. Owing to the heavy bentonite slurry having a highspecific gravity, the bentonite slurry in the rock-bed can surely beheld in a dual structure having a light bentonite slurry as an upperlayer, and a mud cake can smoothly be formed at the top end part of therock-bed cavity, which is a weak part where the void and the crack arehardly blocked up, by closely contacting the light bentonite slurry withthe top end part easily.

[0014] It is preferred that in order not to lose activity by maintaininga stable state for a long period of time, the bentonite slurry is addedwith a dispersing agent such as phosphate, lignite, styrensulfonicacid/maleic anhydride copolymer and polyacrylic acid in accordance withnecessity.

[0015] The light bentonite slurry is, in general, inexpensive comparedwith the heavy bentonite slurry which is expensive. Therefore, due tothe dual structure in which the upper layer of the heavy bentoniteslurry is replaced with light bentonite slurry, economic efficiency canbe obtained by reducing the quantity of use of the heavy bentoniteslurry. In addition, economical maintenance management can be obtainedby supplementing the reduced quantity of the bentonite slurry in therock-bed cavity with the inexpensive light bentonite slurry.

[0016] Moreover, according to the compressed gas storage tank of thepresent invention, since the compressed gas pipe for feeding acompressed gas is open downward from the top part of the rock-bedcavity, the bentonite slurry can fully be filled in the cavity of therock-bed while completely allowing air and the like to escape from thecavity of the rock-bed, and in addition, the void and the crack of thetop end part of the rock-bed cavity, at which part the stored gas tendsto escape, can surely and economically be blocked by closely contactingthe inexpensive light bentonite slurry mixed with a filling-up materialwith the top end part of the rock-bed cavity.

[0017] In the case where the light bentonite slurry is forcibly fed intothe rock-bed cavity, in which the bentonite slurry is filled, throughthe compressed gas pipe, the light bentonite slurry is fed into therock-bed cavity from the top part to the ceiling part of the rock-bedcavity with pressure applied thereto while pushing out the bentoniteslurry already filled therein and therefore, blocking of the void andthe crack formed in the rock-bed is enhanced by efficiently entering thelight bentonite slurry into the void and the crack with the pressure toform the mud cake. Even if the light bentonite slurry is reduced bybeing infiltrated into the rock-bed, the function for blocking the voidand the crack can be maintained continuously and economically bysupplementing the inexpensive light bentonite slurry through thecompressed gas pipe easily.

[0018] Moreover, according to the compressed gas storage tank of thepresent invention, in the case where the vertical shaft has associatedfacilities such as a reverse osmosis membrane water-generating pipe anda deep aeration pipe juxtaposed thereto, economic efficiency can beobtained by effectively utilizing the vertical shaft formed byexcavating the underground deep into the rock-bed cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic sectional view showing a state of storage ofa compressed air in a compressed gas storage tank according to oneembodiment of the present invention;

[0020]FIG. 2 is a schematic sectional view showing a state in which acompressed air is removed and a bentonite slurry is filled in thecompressed gas storage tank according to one embodiment of the presentinvention;

[0021]FIG. 3 is a sectional view taken on line A-A of FIG. 1;

[0022]FIG. 4 is an enlarged view showing the area indicated by B of FIG.1; and

[0023]FIG. 5 is a schematic sectional view showing a compressed gasstorage tank according to another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] One preferred embodiment of the present invention will now bedescribed in detail with reference to the accompanying drawings. Acompressed gas storage tank 10 utilizing a rock-bed cavity according tothis embodiment, as shown in FIGS. 1 to 4, includes a rock-bed cavity 11formed as an underground cavity at a predetermined depth (for example,600 m depth) in a soft rock ground and extending laterally by aconsiderable length (for example, 1000 m), so that an electric powerstoring compressed air (compressed gas) produced utilizing surpluselectric power during night time can be stored therein.

[0025] Moreover, according to the compressed gas storage tank 10 of thisembodiment, the compressed air is stored in the rock-bed cavity 11 in astate in which the compressed air is loaded from the underside thereofwith a hydrostatic pressure of a heavy bentonite slurry 13 fed into therock-bed cavity 11 through a vertical shaft 12. A light bentonite slurry30 is fed and arranged in such a manner as to cover the upper side ofthe heavy bentonite slurry 13 fed into the compressed gas storage tank10. Owing to difference in specific gravity between the heavy bentoniteslurry 13 and a light bentonite slurry 30, the bentonite slurry in thecompressed gas storage tank 10 is of a dual structure consisting of anupper layer composed of the light bentonite slurry 30 and a lower layercomposed of the heavy bentonite slurry 13.

[0026] Moreover, according to the compressed gas storage tank 10 of thisembodiment, the rock-bed cavity 11 is constructed such that its ceilingpart 15 exhibits a rising gradient toward its top part 14 which is aconnecting portion with the vertical shaft 12 and a compressed gas pipe16 for feeding a compressed air to the rock-bed cavity 11 is opendownward from the top part 14.

[0027] The rock-bed cavity 11 is an underground cavity having agenerally circular section which has a diameter of about 10 to 15 m. Thecavity 11 is formed by excavating the rock-bed laterally so as to have alength of about 500 m on both left and right sides from the lower endportion of the vertical shaft 12 by means of various kinds of knownrock-bed tunnel excavating method. The rock-bed cavity 11 is generallyhorizontally formed and its diameter becomes smaller towards the outsidefrom its part located below the vertical shaft 12. Owing to thisfeature, its ceiling part 15 exhibits a gentle rising gradient towardsits top part 14 where a lower end part of the compressed gas pipe 16 isopen. A liquid pool portion 17 in the shape of a recess is formed, byenlarged excavation operation, in a central bottom area of the rock-bedcavity 11 located below the top part 14. A lower end of a mud feed pipe18, which is pipe-arranged along the vertical shaft 12 and allowed toextend across the rock-bed cavity 11 so as to dent downward, is arrangedto be open to the liquid pool portion 17.

[0028] The vertical shaft 12 is a vertical hole having a circularsection which has a diameter of about 6 m, formed by digging theunderground formation from the ground surface vertically downward priorto the excavation operation of the rock-bed cavity 11, using variouskinds of known vertical hole excavating methods. A mud feed pipe 18extends through the vertical shaft 12. By supplying the heavy bentoniteslurry 13 into the mud feed pipe 18, it can be fed into the rock-bedcavity 11 through the vertical shaft 12.

[0029] The mud feed pipe 18 is a steel pipe having a circular sectionwhich has a diameter of about 2 m. The mud feed pipe 18 is inserted intothe vertical shaft in such a manner as to sequentially join a componentpipe element to an upper end of the preceding pipe element and to insertsuch pipe elements assembly downward from the ground surface. A lowerend of the mud feed pipe 18 is disposed in the liquid pool portion 17. Agrout material 20 is supplied between an external side of the mud feedpipe 18 disposed within the vertical shaft 12 and an inner wall surfaceof the vertical shaft 12 in such a manner as to fill a gap therebetweenand hardened. By doing so, the inner surface of the vertical shaft 12 isprotected, and the mud feed pipe 18 is firmly fixed within the verticalshaft 12 (see FIG. 3). A concrete-made blocking plug 19 is formed at aconnecting portion between a lower end part of the vertical shaft 12 andthe rock-bed cavity 11 in such a manner as to cover the periphery of themud feed pipe 18 (see FIG. 4). By virtue of a provision of the blockingplug 19, the compressed air and the light bentonite slurry 30accumulated in the rock-bed cavity 11 are prevented from leaking upwardthrough the gap between the inner wall surface of the vertical shaft 12and the mud feed pipe 18. A lower end face of the blocking plug 19constitutes the top part 14 which occupies the highest position of theceiling part 15.

[0030] The compressed gas pipe 16 for feeding a compressed air into therock-bed cavity 11 and a reverse osmosis membrane water-generating pipe16 are disposed along the mud feed pipe 18. The compressed gas pipe 16is a pipe made of FRP (fiber reinforced plastics) having a diameter ofabout 100 mm. One end of the compressed gas pipe 16 is connected to acompressor disposed at an electric power generating facility 26 as laterdescribed, and the other end is once allowed to project outward of themud feed pipe 18 at a lower end part of the mud feed pipe 18 and thenembedded in the blocking plug 19 with a distal end opening thereof beingopen downward from the lower end face of the blocking plug 19 which isthe top part 14 of the rock-bed cavity 11.

[0031] The reverse osmosis membrane water-generating pipe 21 isconstituted by installing a reverse osmosis membrane module 23 taught byJapanese Patent Application Laid-Open No. 10-156356 to a lower end partof a protective pipe 22 composed of a steel pipe having a circularsection which has a diameter of about 1 m. In reverse osmosis membranewater-generating pipe 21, a salt water composed of, for example, seawater, is supplied to the inside of the protective pipe 22, and a freshwater collected into a production water vessel via a fresh watercollection pipe of the reverse osmosis membrane module 23 is lifted upby a lifting pump through a lifting pipe 24. By doing so, fresh water isproduced. According to this reverse osmosis membrane water-generatingpipe 21, if water in the fresh water collection pipe is lifted up, apressure difference equal to or larger than the reverse osmosis pressurecaused by the salt water in the protective pipe 22 is normally generatedin a natural condition between the internal pressure thereof and that ofthe fresh water collection pipe due to hydrostatic pressure applied toan outer periphery of the reverse osmosis membrane module 23 andtherefore, a pressure difference necessary for reverse osmosis caneasily be obtained. This makes it possible to produce fresh watereconomically and efficiently. According to this embodiment, a lower endpart of the protective pipe 22 of the reverse osmosis membranewater-generating pipe 21 projects downward beyond a lower end of the mudfeed pipe 18 and its distal end is supportingly embedded in a bottompart of the liquid pool portion 17. Accordingly, the reverse osmosismembrane water-generating pipe 21 is installed along the mud feed pipe18 in a stable manner.

[0032] Moreover, according to this embodiment, a heavy bentonite slurrystorage reservoir 25 for storing the heavy bentonite slurry 13 to be fedto the rock-bed cavity 11 is formed on the ground surface adjacent tothe vertical shaft 12. This heavy bentonite slurry storage reservoir 25has an electric power generating facility 26 equipped with thecompressor generating the compressed air, turbine generator, etc. Theelectric power generating facility 26 is disposed on the reservoir 25 ina floating state. The upper end opening of the mud feed pipe 18 isconnected with the heavy bentonite slurry storage reservoir 25.

[0033] According to the compressed gas storage tank 10 of thisembodiment, prior to operation of the tank 10, the work forair-tightening the rock-bed cavity 11 is carried out by blocking thevoid and the crack formed in the inner wall surface of the rock-bedcavity 11. That is to say, after water is fed into the rock-bed cavity11 to clean it, the heavy bentonite slurry 13 having a specific gravityof 1.20 to 2.0 is fed through the mud feed pipe 18 so as to fill therock-bed cavity 11 and the mud feed pipe 18. Then, the light bentoniteslurry 30 having a specific gravity of 1.05 to 1.20 is forcibly fed intothe rock-bed cavity, in which the heavy bentonite slurry 13 has beenfilled, through the compressed gas pipe 16 in such a manner as to pushout the heavy bentonite slurry.

[0034] The heavy bentonite slurry 13 is a suspension liquid obtained bymixing a muddy water with high specific gravity fine powder such asbarite and hematite as a load adjusting material in such a stablecondition that it may hardly precipitate. The heavy bentonite slurry isa comparatively expensive muddy water having a specific gravity of about1.2 to 2.0. Specifically, a bentonite slurry having a weight mix ofTable 1, for example, can be used as the heavy bentonite slurry 13.TABLE 1 Composition of heavy bentonite slurry Specific gravity/material2.0 1.8 1.6 1.4 1.2 Base material 100 100 100 100 100 fresh water (ml)Viscosity improver 8.5 8.2 8.0 8.0 8.0 bentonite (g) Viscosity improver0.20 0.20 0.20 0.15 0.10 driscal (g) Dispersant 0.50 0.45 0.20 0.10 0.05SSMA (g) Load adjusting 183.3 136.3 89.2 56.3 22.8 material barite (g)Alkari agent 0.10 0.16 0.16 0.10 0.10 slaked lime (g) Antiseptic agent0.05 0.05 0.05 0.05 0.05 HCHO (g) Evaporation inhibitor little littlelittle little little liquid paraffin Total capacity (ml) 145 135 125 115107

[0035] The light bentonite slurry 30 is a muddy water having a specificgravity of 1.05 to 1.20 obtained, for example, by adding powder ofcalcium carbonate having an average grain diameter of about 10 to 40microns as a filler while slightly increasing the quantity of bentoniteas a viscosity improver which is to be blended with a bentonite slurry.This light bentonite slurry 30 is inexpensive compared with the heavybentonite slurry 13. The light bentonite slurry 30 is mixed with, forexample, a lost circulation inhibitor (LCM) as a filling-up materialserving as a core at the time of forming a mud cake for blocking thevoid and the crack formed in the rock-bed. Owing to presence of thefiller and the filling-up material, the mud cake is surely and firmlyretained in the void and the crack formed in the rock-bed. Specifically,a bentonite slurry having a weight mix of Table 2, for example, can beused as the light bentonite slurry 30. As the lost circulationinhibitor, there can be used plants such as cottons, walnut shells, pulpprocessed products and the like, minerals such as pulverized substancesof vermiculites, mica pieces, chrysotiles and the like, or syntheticproducts such as cut articles of film, mixtures of cellophane and thelike. Particularly, in order to rapidly fill up the void and the crackformed in the ceiling part 15 of the rock-bed cavity 11 where leakage ofthe compressed gas tends to occur, it is preferable to use a material,such as a light weight aggregate which can be floated and suspended atan upper part in the bentonite slurry. TABLE 2 Composition of lightbentonite slurry Specific gravity/material 1.15 to 1.10 Base material100 fresh water (ml) Viscosity improver 10.0 bentonite (g) Filler 8.0calcium carbonate powder (g) Dispersant 0.05 SSMA (g) Load adjustingmaterial (g) — Alkari agent (g) — Antiseptic agent 0.05 HCHO (g)Evaporation inhibitor little liquid paraffin Total capacity (ml) 107

[0036] By filling the heavy bentonite slurry 13 and the light bentoniteslurry 30 in the rock-bed cavity 11 in this way, a mud cake is formed onthe inner wall surface of the rock-bed cavity 11 and the void and thecrack of the rock-bed are blocked with the mud cake. Thus, the rock-bedcavity 11 is air-tightened. According to this embodiment, since thebentonite slurry filled in the rock-bed cavity 11 has a dual layerstructure consisting of the heavy bentonite slurry 13 and the lightbentonite slurry 30 and the light bentonite slurry 30 mixed with thefilling-up material is closely contacted in a pressurized condition withthe ceiling part 15 of the rock-bed cavity 11 where the compressed gastends to easily leak outside, a mud cake is firmly and surely formed bythe light bentonite slurry 30 and therefore, the void and the crackformed in the ceiling part 15 can surely be blocked with the mud cake.

[0037] Moreover, according to this embodiment, the rock-bed cavity 11serves its connecting portion with the vertical shaft 12 as the top part14, towards which the ceiling part 15 of the rock-bed cavity 11 exhibitsa rising gradient, and the compressed gas pipe 16 is open downward fromthe top part 14. Accordingly, during the air-tightening operation of therock-bed cavity 11, the void and the crack formed in the ceiling part 15where the stored compressed air tends to easily leak outside can be alsosurely blocked by completely filling the light bentonite slurry 30 andthe heavy bentonite slurry 13 into the rock-bed cavity 11, whileallowing air to completely escape from the rock-bed cavity 11 withoutremainder. Thus, a firm air-tightening performance can be given to therock-bed cavity 11.

[0038] Furthermore, according to the compressed gas storage tank 10 ofthis embodiment, after the rock-bed cavity 11 is air-tightened by theheavy bentonite slurry 13 and the light bentonite slurry 30, thecompressed gas storage tank 10 is operated. That is to say, compressedair is produced by the compressor within the electric power generatingfacility 26 utilizing surplus electric power at night time, for example.And the compressed air thus produced is forcibly fed into the rock-bedcavity 11 through the compressed gas pipe 16. The forcibly fed compressair is stored in the compressed gas storage tank in a state in which itis loaded with a hydrostatic pressure load from its underside by theheavy bentonite slurry 13, while pushing down the heavy bentonite slurry13 and the light bentonite slurry 30 in the rock-bed cavity 11 by thepressure. The pushed-down heavy bentonite slurry 13 is sequentiallyflowed into the mud feed pipe 18 the lower end opening of which is opento the liquid pool portion 17, as the capacity of the compressed air isincreased in the rock-bed cavity 11. And the heavy bentonite slurry 13is then moved to the heavy bentonite slurry storage reservoir 25 throughthe mud feed pipe 18 in such a manner as to be pushed up (see FIG. 1).

[0039] Since the lower end opening of the mud feed pipe 18 is opened tothe liquid pool portion 17 formed in the shape of a recess at a locationeven lower than bottom part of the rock-bed cavity 11, the lower endopening of the mud feed pipe 18 is kept inserted into the heavybentonite slurry 13 and the light bentonite slurry 30 remained in theliquid pool portion 17 so that the compressed air is prohibited fromflowing therein, even if the compressed air is stored so far as to thebottom part of the rock-bed cavity 11. Accordingly, the compressed aircan be efficiently stored in large capacity as far as to the bottom partof the rock-bed cavity 11.

[0040] On the other hand, in day time, for example, when a largequantity of electric power is required, the stored compressed air istaken out of the rock-bed cavity 11 into the electric power generatingfacility 26 through the compressed gas pipe 16, and fuel and air areadmixed so as to be combusted under high pressure, thereby causing theturbine of the turbine generator to rotate so as to generate an electricpower. As the capacity of the compressed air is reduced due totaking-out of the compressed air, the heavy bentonite slurry 13 movedfrom the heavy bentonite slurry storage reservoir 25 is supplemented inthe rock-bed cavity 11 and the compressed air is still kept pushed fromits underside by the hydrostatic pressure of the heavy bentonite slurry13. In a state in which the compressed air is completely discharged fromthe rock-bed cavity 11, the heavy bentonite slurry 13 is filled in therock-bed cavity 11 again in the presence of the upper layer composed ofthe light bentonite slurry 30 at the ceiling part.

[0041] In this way, the storage and taking-out of the compressed air arerepeatedly carried out by operation of the compressed gas storage tank10. In association with this repeating procedure, an air-tight statecaused by filling of the compressed air and a liquid-tight state causedby filling of the bentonite slurry appear repeatedly in an upper halfportion of the rock-bed cavity 1 1. Due to repetition of the air-tightstate and the liquid-tight state (metasomatism of liquid-tightness andair-tightness), the mud cake grows and is consolidated and hardened toallow self-recovery.

[0042] According to this embodiment, since the light bentonite slurry 30mixed with the filling-up material is closely contacted with the ceilingpart 15 of the rock-bed cavity 11 where the compressed gas tends toeasily leak outside, a firm and stable mud cake is self-recovered by thelight bentonite slurry 30, thus making it possible to more surely blockthe void and the crack formed in the ceiling part 15. Moreover, sincethe ceiling part 15 of the rock-bed cavity 11 exhibits a rising gradienttowards the top part 14 and the compressed gas pipe 16 is open downwardfrom the top part 14, the ceiling part 15 where the stored compressedgas tends to easily escape outside can surely be self-recovered bysurely creating a liquid-tight state up to the ceiling part 15 and thetop part 14 of the rock-bed cavity 11 while allowing the compressed gasto escape completely from the rock-bed cavity through the compressed gaspipe 16.

[0043] That is to say, according to the compressed gas storage tank 10of this embodiment, a sufficient air-tightness can be obtained by surelyblocking the ceiling part 15 of the rock-bed cavity 11, and theself-recovering operation, which is carried out while repeating theliquid-tight state and the air-tight state, can surely be performed atthe ceiling part 15 of the rock-bed cavity 11. Therefore, and thecompressed gas can surely be stored in a stable manner without allowingit to escape.

[0044] Moreover, since the reverse osmosis membrane water-generatingpipe 21 arranged at the interior of the mud feed pipe 18 is juxtaposedto the vertical shaft 12, economic efficiency can be obtained byeffectively utilizing the vertical shaft 12 which is formed byexcavating the underground deep to the rock-bed cavity 11.

[0045] Furthermore, since the bentonite slurry to be filled into therock-bed cavity 11 has a dual layer structure consisting of the heavybentonite slurry 13 and the light bentonite slurry 30, and the quantityof use of the expensive heavy bentonite slurry 13 can be reduced by aportion equal to the quantity of use of the light bentonite slurry 30,economic efficiency can be obtained.

[0046] Moreover, since the supplement made at the time of reduction ofthe bentonite slurry is carried out utilizing the inexpensive lightbentonite slurry 30 which is fed through the compressed gas pipe 16, themaintenance management of the compressed gas tank 10 can be performedeconomically.

[0047] It should be noted that the present invention is not limited tothe above-mentioned embodiment and many changes can be made. Forexample, the rock-bed cavity 11 is not necessarily obtained by formingan underground cavity by means of tunnel excavation. Instead, aabandoned mine and a natural cavity can be used directly as the rock-bedcavity 11 only by giving some correction, where necessary. Similarly,the vertical shaft is not limited to the juxtaposition to the reverseosmosis membrane water-generating pipe. Instead, a hot water storagepipe, an ice heat accumulation pipe, a deep aerating pipe, a sludgedigestive pipe, a sewage complete aerating pipe and the like may bejuxtaposed. Moreover, other compressed gas than the compressed air canbe stored.

[0048] It is not absolutely necessary that the compressed gas pipeopening downward from the top part of the rock-bed cavity is arrangedwithin the vertical shaft and its lower end is open at the connectingportion between the vertical shaft and the rock-bed cavity. For example,as shown in FIG. 5, it is also accepted that a compressed gas pipe 33 isdisposed towards the top part 34 of the rock-bed cavity 32 separatelyfrom the mud feed pipe 31 for feeding the heavy bentonite slurry 13 andthe compressed gas pipe 33 is open downward from the top part 34.

Industrial Applicability

[0049] According to a compressed gas storage tank utilizing a rock-bedcavity of the present invention, a sufficient air-tightness can beobtained by surely blocking the ceiling part of the rock-bed cavity, andthe self-recovering operation, which is carried out while repeating theliquid-tight state and the air-tight state, can surely be performed atthe ceiling part of the rock-bed cavity. Therefore, the compressed gascan surely be stored in a stable manner without allowing it to escape.

1. A compressed gas storage tank utilizing a rock-bed cavity in which abentonite slurry is fed into an underground cavity formed in a rock-bed,and a compressed gas forcibly fed to said underground cavity through acompressed gas pipe is stored in said rock-bed cavity in a state inwhich the compressed gas is loaded with a pressure load of the bentoniteslurry from the underside of the compressed gas, wherein said compressedgas pipe is open downward from a top portion of said rock-bed cavity,and said bentonite slurry in said rock-bed cavity is of a dual layerstructure consisting of an upper layer composed of a light bentoniteslurry mixed with a filling-up material invading into and filling up avoid and a crack formed in an inner wall surface and having a specificgravity of 1.05 to 1.20, and a lower layer composed of a heavy bentoniteslurry mixed with a high specific gravity fine powder as a loadcondition material and having a specific gravity of 1.20 to 2.0.
 2. Thecompressed gas storage tank utilizing a rock-bed cavity according toclaim 1, wherein said light bentonite slurry is forcibly fed throughsaid compressed gas pipe into said rock-bed cavity in which thebentonite slurry is filled.
 3. The compressed gas storage tank utilizinga rock-bed cavity according to claim 1, wherein said rock-bed cavity isan underground cavity extending laterally with a ceiling part thereofexhibiting a rising gradient toward said top part which is a connectingportion with said compressed gas pipe.
 4. The compressed gas storagetank utilizing a rock-bed cavity according claim 1, wherein the heavybentonite slurry is fed into said rock-bed cavity through a verticalshaft formed in the underground and said vertical shaft has associatedfacilities such as a reverse osmosis membrane water-generating pipe anda deep aeration pipe juxtaposed thereto.